Certain applications of polymers require conductive, semi-conductive, or static charge-dissipating properties.
Electrically conductive polymer products have been made by mixing an electrically conductive material into the polymer during formation. Examples of such electrically conductive materials include powders such as silver, copper, nickel, carbon black, and graphite; and ionizable metal salts such as alkali metal tetraorganoborides. To provide satisfactory conductive properties, the amount of conductive powder in a conductive material can be as high as 10-40% by weight, which can affect the thermal and mechanical properties of the polymer. Uniform high conductivity requires uniform distribution of the conductive material throughout the polymer bulk.
In one aspect, the invention features a conductive quasi-solution useful for making a homogeneously conductive single-phase polyurethane. The quasi-solution includes between 10% and 30% by weight of a transition metal salt, a carrier solution, a dispersing agent. The carrier solution includes a polyol, a polyamine, or a flame retardant. In preferred embodiments, the carrier solution includes a flame retardant.
In another aspect, the invention features a method of making a homogeneously conductive single-phase polyurethane, including the steps of preparing a conductive quasi-solution and mixing an amount of the conductive quasi-solution with a diisocyanate and a polyurethane precursor to form a homogeneously conductive single-phase polyurethane (i.e., a solid solution). The polyurethane precursor is selected from polyols and polyamines.
Mixing is accomplished according to either a conventional reaction injection molding process, or a conventional thermoplastic polyurethane production process.
The conductive thermoset and thermoplastic polyurethanes have many applications. Examples include coated integrated circuit boards, car body parts, especially those which are to have a painted (baked) surface, such as bumpers and side panels; and durable carrying cases or other accessories in the electronics (e.g., computer) field for which dissipation of static charge is desirable.
The transition metal salt provides conductive properties to the polyurethane. As used herein, homogeneously conductive means that conductivity is essentially uniform across the surface and throughout the volume of the bulk polyurethane, due to the uniform distribution of fully-dissolved transition metal salts throughout the polymer. The polymer is thus a single-phase solid solution of polymer and dissolved transition metal salts, with an absence of transition metal salt particles. The methods of the invention produce single-phase conductive polyurethanes, which are therefore homogeneously conductive.
Other features and advantages of the invention will be apparent from the detailed description, and from the claims.
The invention generally features the use of a conductive quasi-solution to make a homogeneously conductive, single phase polyurethane. The conductive quasi-solution is a dispersion including a transition metal cation, a dispersing agent, and a carrier. A conductive quasi-solution has both dissolved and undissolved transition metal salt, the maximum diameter of undissolved particles being 0.5 xcexcm, and preferably 0.1 xcexcm.
The invention resides, in part, in the discovery that a quasi-solution of transition metal salts will produce a conductive single-phase polyurethane product. This product, whether formed by reaction injection molding or conventional thermoplastic polyurethane production, is a solid solution, rather than a dispersion of undissolved transition metal salts in the polymer. Upon mixing, the undissolved transition metal particles dissolve into the forming polymer. For example, mixing a copper (II) chloride quasi-solution (a brown dispersion with undissolved copper salt particles) with the urethane precursor(s) results in a clear, dark green solution without any precipitated or undissolved copper salt particles. Similarly, mixing an iron (III) chloride quasi-solution (a dark grey dispersion) with the urethane precursor(s) results in a clear, light brown solution without any precipitated or undissolved particles of iron salt. The reaction of the precursor with a diisocyanate forms a uniform, light green polyurethane product where a copper quasi-solution is used; a uniform light brown polyurethane product is formed where an iron quasi-solution is used. These color changes indicate that the polyurethane product is a solid solution, i.e., a homogeneous, single-phase polymer.
In addition, the transition metal cations form d-orbital complexes with nucleophilic oxygen (i.e., carbonyl) and nitrogen atoms. Such complexes form bridged networks between different polyurethane polymer molecules, which improves uniformity and conductivity.
A transition metal salt is capable of carrying or conducting an electric charge. According to the invention, a transition metal salt includes a transition metal cation and one or more counter-anions to form a neutral salt. Each counter-anion may be any anion that does not interfere with the homogeneity or semi-conductivity of the final polymer product. Examples of transition metal cations include Cu (II), Co (II), Zn (II), Ni (II), Fe (II), and Fe (III). Fe (III) and Cu (II) are preferred. Examples of counter-anions include fluoride, chloride, bromide, iodide, cyanide, thiocyanate, acetate, lactate, phosphate, and tartrate. Thiocyanate, bromide, chloride, and acetate are preferred. Thus, examples of transition metal salts include CuCl2, CuBr2, Cu(CH3COO)2, Cu(SCN)2, FeBr3, Fe(SCN)3, FeCl2, CuCl2, and Fe(CH3COO)3. Transition metal salts are available commercially from, e.g., Aldrich Chemical Co. (Milwaukee, Wis.).
The conductive quasi-solution preferably has a transition metal salt concentration (w/w in carrier and dispersing agent) of between 10 and 30%, and more preferably between 15 and 25%. Generally, in the absence of a dispersing agent, the solubility of a transition metal salt in a carrier is between 5 and 10% (w/w). The concentration (w/w) of transition metal salts in the final product is between 0.02 and 1.0%, and is preferably between 0.04 and 0.2% (see, e.g., Example 12). Thus, the mole percent of transition metal salts in the final product is between 1.52xc3x9710xe2x88x924 and 7.4xc3x9710xe2x88x923 mole percent, and preferably between 3.0xc3x9710xe2x88x924 and 1.5xc3x9710xe2x88x923 mole percent. In some embodiments, more than one transition metal salts are used; as a result, more than one transition metal cation or more than one counter-anion is present in the conductive quasi-solution.
According to the invention, a carrier solution is one or more organic solvents, the combination solution being stable at high temperature. Such a temperature-stable non-aqueous liquid is characterized by a high boiling point and a low vapor pressure. A high boiling point is a boiling point of at least 350xc2x0 F., and preferably at least 500xc2x0 F. A low vapor pressure is a vapor pressure of no more than 50 mm Hg at 110xc2x0 C. A low vapor pressure is also characterized by the visual absence of white smoke at 300xc2x0 F. Examples of carrier solutions include flame retardants such as tri(haloalkyl)phosphates, and preferably tri(chloro-ethyl)phosphate (xe2x80x9cFyrol(copyright) CEF,xe2x80x9d Akzo Chemical Corp., Chicago, Ill.) and tri(xcex2-chloroisopropyl)phosphate (xe2x80x9cPCFxe2x80x9d); polyamines such as Polamine(copyright) 650, Polamine(copyright) 1000, and Polamine(copyright) 2000 (Air Products, Allentown, Pa.); and polyols such as polypropylglycol (xe2x80x9cPPGxe2x80x9d), Terathane(copyright) 650, Terathane(copyright) 1000, Terathane(copyright) 2000, Terathane(copyright) 2900, Terathane(copyright) CL-2000 (Du Pont de Nemours, Wilimington, Del.), and Pluraco(copyright) Polyol 924, Pluracol(copyright) Polyol 538, and Pluracols(copyright) Polyol 774 (BASF Corp., Parsippany, N.J.), and butanediol XB (GAF Corp., Wayne, N.J.)
In some embodiments, more than one solvent is present in the conductive quasi-solution, such as the following carrier combinations: a polyol and a polyamine; two polyols; two polyamines; a flame retardant and a polyol; a flame retardant and a polyamine; or a flame retardant, a polyol, and a polyamine.
According to the invention, a dispersing agent is a non-ionic surfactant which acts to stabilize the metal salt in the conductive quasi-solution before use in a reaction injection molding process. After mixing, the dispersing agent also improves solubility of the transition metal salt, improves uniform mixing, and thus improves the conductive homogeneity of the final product. Stabilization of the conductive quasi-solution includes increasing the solubility of the transition metal salt without increasing the percent of carrier. Examples of dispersing agents include DC 190 and DC 193 (Dow Corning Corp., Midland, Mich.), DABCO(copyright) DC 5418 (Air Products Co.) and RC Dispersant WM (Rhein Chemie Corp., Trenton, N.J.). Dispersing agents are present in the quasi-solution at a concentration between 0.5 and 5%, and preferably 1 and 3% (w/w).
The conductive quasi-solution of the invention may be used in either a two stream process (streams A and B) or a three stream process (streams A, B, and C) to produce a molded homogeneously conductive polyurethane form. The mixing step may be part of either a conventional reaction injection molding process or a conventional production method of making thermoplastic polyurethane. A diisocyanate can be in prepolymer form (i.e., partially cured with a polyol or polyamine) or pure form, such as methylene diisocyanate (MDI) or toluene diisocyanate (TDI) (BASF Corp.).
In a two stream process, stream A includes a diisocyanate in either a prepolymer form such as the commercially available Bayflex(copyright) 110-25A (Bayer Corp, Pittsburgh, Pa.) or pure form, such as Isocyanate(copyright) 76 and Isocyanate(copyright) 227 from BASF. Stream B includes a conductive quasi-solution at a concentration between 0.2 to 2.0% by weight, and a polyurethane precursor such as a polyol or a polyamine in a solvent such as Bayflex(copyright) 110-35B, IMR (Bayer Corp.). Stream B also may include a catalyst such as an amine catalyst, a tin catalyst, or a bismuth catalyst, in a concentration between 0.01 and 0.5%, and preferably between 0.02 and 0.3% by weight with respect to the total amount of polyol and polyamine. Examples of catalysts include DABCO(trademark) 33-LV, DABCO(trademark) T-12 and DABCO(trademark) T-9 (Air Product Co.) and Fomrez(copyright) UL-32 (Witco, New York, N.Y.).
In a three stream process, stream A includes a diisocyanate as above. Stream B includes a polyurethane precursor such as a polyol or a polyamine. In general, the total concentration of polyol and polyamine is at least 95%, the remainder being additives such as lubricants (e.g., internal mold release materials), catalysts, pigments, mineral additives (e.g., glass fibers, iron oxide, carbon black, or graphite). Stream C includes the conductive quasi-solution. Stream C may also include additives such as pigment.
In either process, the streams are mixed at a specified flow rate ratio, temperature, and pressure at the mixing head and rapidly injected into the mold, e.g., within 1 or 2 seconds. A person of skill in the art will easily be able to determine the specific ratios, temperatures, and pressures to be used. See, e.g., Example 12.
In general, the conductive quasi-solution of the invention has a very short pot life (e.g., less than 5 seconds). After contact with a polyurethane precursor such as a polyol or a polyamine or a combination thereof, the previously-undissolved transition metal salt particles in the conductive quasi-solution completely dissolve. Moreover, once the streams are in contact with each other, a single-phase solid solution polyurethane is formed. The concentration (w/w) of the quasi-solution in the final product is between 0.1 and 5 % and preferably between 0.2 and 1.0%.